In modern papermaking, an ever-increasing emphasis is being placed on more complete recovery of furnish materials, such as pigments, fibers and additives, as a part of the ultimate sheet. Some of the many benefits the papermaker realizes by increasing the retention, especially of the fine particulate matter associated with respect to these components, are:
(a) better economic utilization of the furnish materials. Obvious economic advantages are achieved by increasing the retention of the furnish materials on the wire of the paper-making machine to provide more paper product and less waste; PA1 (b) improved quality of the resultant sheet product. The retention of supplied furnish material yields a more uniform grade of product and fewer rejects on the production end product; PA1 (c) less down-time due to a cleaner system. The retention of most of the fiber, fines, fillers and additives on the web does not allow these materials to accumulate in the recirculating white water of the production system. In systems where the white water is at least partially recirculated, this provides less build-up or concentration of fines in the the head box. It also provides less solids in the effluent stream and thereby minimizes the need for extensive treatment of the stream before release; PA1 (d) increased production. By providing a means to retain the solids on the web and provide an increased drainage rate, a paper making machine can be frequently increased in rate and thereby increase production; and PA1 (e) more versatile paper production. The ability to retain more solids on the web can often reduce two-sidedness of the paper product and provides a means of incorporating materials otherwise impossible to use.
A large variety of materials have been used as retention aids. The most widespread of these, and among the oldest in use, are the salts of aluminum, in particular aluminum sulfate, sodium aluminate and sodium phosphoaluminate. These materials, however, have the defects of being required to be used in large amounts and not be a highly efficient retention aid.
Various polymeric materials, from naturally occurring gums to synthetic resins have also been used as retention agents. These include natural and chemically modified starches. The starches are normally retained in the sheet product. Synthetic, water-soluble polyelectrolytes which are cationic or anionic have also been used. Anionic polyelectrolytes are generally of the polyacrylamide type which has been partially hydrolyzed to thereby contain from 5 to 30 mole percent carboxyl groups.
The cationic polymers found useful are ones which generally contain cationic nitrogen-containing groups which may be in the form of free amino groups or of quaternary ammonium salts. These polymers are generally found useful for paper making processes using alkaline conditions. Typical polymers used for this purpose include polyvinyl imidazolines, polyaminoalkyl acrylates, polyethyleneimines, polyamideamines, copolymer of acrylamide and methacryloyloxyethyl trimethyl ammonium salts and co- and terpolymers of dialkyl diallyl ammonium halides. These and similar polymers are discussed in Structure/Performance Relationship of Cationic, Anionic and Amphoteric Retention Aids by Joseph Watts, Jr. et al, TAPPI 25th Paper Conference, pg., 53-67 (1970) as well as in U.S. Pat. Nos. 2,585,583; 3,544,532; 3,679,748; 3,772,076. These polymers are deficient because of their low charge density and/or low molecular weight.
It is desired to obtain a polymeric material which has a high charge density, has a molecular structure capable of providing a high degree of attraction between polymer and particulate materials commonly contained in paper furnish and has a molecular weight range capable of aiding in the retention of particulate material on the web.